Here are my recommendation:
Starter: Popplio I have never used it but it has a useful typing and good IVs.You also get surf very early.
Other pokemon I am going to recommend are:
Chandelure: flash fire is very useful.Shadow ball,flame burst,pain split are also useful.It can also learn some powerful Tms.
Mamoswine: Thick fat ability.Breed with Alolan Sandslash for icicle crash.With moves like earthquake and ice shard very powerful.
Tyrantum: Rock head ability.breed with altaria for dragon dance.It can learn head smash,earthquake,dragon claw.
Swoobat: Simple abilty.With calm mind,confusion,air slash and later psychic,you won't have to worry about anything early game.
Alolan Raichu: Nasty plot,thunderbolt and psychic.
Don't use rowlet because it learns steam eruption instead of spirit shackle because of a bug.
There is also a mystery egg in the game.